Subdivision Surface Modifier Mask?
The basic problem you have is the topology. Avoid faces that have more than 4 vertices. Those will not work for what you want to do.
All of those examples don't work because of those n-gons.
Start with a shape like this:

You can create all kinds of &quot;unusual shapes&quot;, but you have to have the topology in mind when creating them.
Then you will be able to appreciate the difference between all of the proposed ways to mix smooth and hard edges.